a) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device including a step of depositing a silicon film.
b) Description of the Related Art
A silicon film doped with impurities is electrically conductive and can be used as a conductive member. Impurities may be doped, at the same time when a film is formed, or by ion implantation or diffusion after the film is formed.
Silicon films doped with impurities are used as gate electrodes of MOSFETs, parts of polycide (lamination of polycrystalline silicon and silicide) gate electrodes, capacitor electrodes of DRAMs, wirings, and parts of polycide wirings. The resistivity of a silicon film used as a conductive member is preferably made as low as possible.
The resistivity depends on the mobility of carriers and the carrier concentration. It is more preferable to use polycrystalline silicon than amorphous silicon in order to increase a carrier mobility. Although a higher mobility is obtained by using single crystal silicon, it is generally and practically difficult to form a single crystal silicon film on the surface of a substance other than silicon. A carrier concentration is generally proportional to a doped amount of impurities, and is limited by a solid solubility of an impurity element in silicon. Therefore, in order to form a silicon film having a low resistivity, a polycrystalline silicon film doped with impurities to a concentration near a solid solubility limit is formed.
Phosphorus is most used as n-type impurities of a silicon film and borons as p-type impurities. In a DRAM, a memory cell unit is formed by n-type MOSFETs and a peripheral circuit unit is formed by CMOS circuits of n- and p-channel MOSFETs. Phosphorus as n-type impurities is therefore most important.